The invention relates to a method for positioning a wafer with a reference marker in a vacuum process installation according to claim 1.
In modern vacuum process facilities circular flat substrates or workpieces, which are also referred to as wafers, are surface-treated, such as for example coated, etched, cleaned, thermally treated etc., in such fully automated vacuum process systems. In order to automate such processes and to be able to carry out multi-stage processes in different facility areas, automated transport systems, a type of handling robot, are employed. In particular the treatment of semiconductor wafers in such processes requires very high quality of treatment, such as in particular high cleanliness, high precision and careful treatment of the substrates. Due to the stated high requirements, such facilities preferably include a lock chamber, where the wafers are moved from the atmospheric environment into a vacuum chamber and subsequently into a process station or, as a rule, sequentially into several process stations in order to be able to carry out the required surface treatment. With the aid of a transport device the wafers are delivered from the lock chamber into the process chamber in a horizontal transport plane, and after the wafer has been deposited in the process chamber, the latter is, as a rule, closed in order to be able to carry out the process under the required vacuum and process conditions. If several process steps are necessary, the wafer is again transported out of the one process chamber in the same manner and, for the next process step, is transported into another process chamber.
Especially preferred types of facilities are so-called cluster systems. In such systems, the lock chamber and the process chamber, or the several chambers, are arranged peripherally about the substantially central transport chamber. In the case of more than one lock chamber and in particular in the case of several process chambers, these chambers are arranged in a type of star-shaped configuration about the centrally located transport chambers. The transport device in this case is located in this centrally located transport chamber and has access, on the one hand, to the at least one lock chamber and, on the other hand, to the process chamber. Between the transport chamber and the remaining chambers conventionally and preferably a so-called lock valve is disposed in order to be able to partition the chambers against one another during the locking process or during the process step. During the transport process of a wafer, the transport device subsequently extends appropriately through the open lock gates in order to deposit the wafer at the designated location.
The transport device moves the wafer translatively in one plane and consequently in two directions of motion. In said preferred cluster systems with the transport device disposed in the central transport chamber, the device is conventionally formed as a mechanism which pivots about a center of rotation and forms therewith the one rotating direction of motion and which can execute a further second translatory motion radially with respect to the center of rotation back and forth away from/to this center of rotation. On this transport device, for example a length-adjustable arm mechanics rotatable in the horizontal plane, the wafer to be transported is subsequently deposited in the end region of this arm. Such a configuration can in this case readily also transport a wafer over relatively great path distances, for example of the orders of magnitude of 1 m or more, from a lock chamber into the transport chamber and from here, in turn, into and out of the process chamber and extend through the corresponding opened lock doors. At the beginning of the transport cycle the wafer is deposited under atmospheric pressure onto the transport mechanism as precisely as possible and always in the same position in order to be able to transport it subsequently also precisely to a predetermined position. However, the deposition of the wafer on the transport mechanism, as well as also the transport mechanism itself, is afflicted with imprecisions or with tolerance errors. Further imprecisions or shifts of the wafer position on the transport mechanism can also occur in the process station due to effects in the process chamber. For this reason the precise position of the wafer must be acquired or measured in order to check the correct wafer position and/or to be able to carry out corrections for the positioning. For this purpose several sensors are conventionally utilized. These sensors are disposed in known manner directly in the proximity of the end position, thus in the processing chamber, where the process is to take place precisely, and subsequently end-positioned in the nominal position. The use of several sensors and the high electronic expenditure together with the positioning process with the transport device leads to very high expenditures and, moreover, therewith, the higher the necessary expenditures, the system trustworthiness or the operational reliability of the vacuum process installation decreases. This can lead to operational failures and stop-downs, to increased maintenance expenditure and also to increased rejects in the production of expensive semiconductor wafers.
Therefore solutions have repeatedly been sought for realizing simpler transport systems with simplified positioning methods in order to decrease expenditures and increase reliability. U.S. Pat. No. 6,760,976 B1 discloses a method for centering a semiconductor wafer, in which a single sensor is utilized instead of several positioning sensors. The method builds on the fact that a circular wafer is utilized, the diameter of which is known and by moving the wafer edge to the sensor at least two points are acquired and with these measuring results, together with the known wafer diameter, the actual position center of the water can be determined. On the basis of this determined center position of the wafer a correction can subsequently be carried out and the wafer can be moved with the transport device into the desired nominal position for the subsequent process step. This method can be applied with circular wafer substrates in which the periphery has a circular closed line and is not perturbed. Semiconductor wafers such as are used today, require a so-called reference marker on the circle periphery, for example a so-called flat, which serves for the circular position detection for the alignment of the structural elements on the wafer and of the wafer itself. As soon as a wafer of this type must be worked, said method leads to errors or failure if edge regions are acquired by the sensor which deviate from the circular shape. For wafers with reference marker this method is therefore not usable.